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A Carbon Signature Revealed

Saturday, May 18, 2013

Imagine diving into the placid surface of a painting by Vermeer, parsing apart Klimt's bejeweled surfaces, or untangling Jackson Pollock's knots of paint. Art historians, collectors, and restoration scholars have long sought to uncover the methods of great painters.

Over the past decade, scientists have peered with light beneath the varnished surface of paintings to discover the chemistry of pigments, to identify the authors of unsigned works, or probe the crack depths from damage or age.

Now, researchers at the University of Barcelona in Spain have used light at terahertz frequencies to uncover the hidden carbon signature of a painting previously thought to be unsigned. Though unsigned, the painting has been studied by art historians and confirmed to be painted by the Spanish artist Goya in 1771. Such secondary validation made the piece an apropos choice by the researchers, who published their findings May 14, 2013 on the arXiv.

"Sacrifice to Vesta" at three different levels of imaging at visible and THz frequencies. Image Credit: http://arxiv.org/abs/1305.3101

Nested between the infrared and microwave regimes, terahertz radiation can travel through materials like plastic or canvas and bounces back slightly differently depending on the chemical composition of each paint color.

To analyze the Goya piece, the researchers parsed the painting into 1 millimeter squares and recorded the reflected terahertz wave from each square, obtaining images with millimeter resolution. To analyze the structure of the painting -- essentially how the paint was laid on the canvas -- the researchers looked at the shape of the reflected wave. At each layer of paint, the reflected wave registered a unique bump. This kind of so-called structural data enabled the team to reveal features of the painting hidden beneath layers of paint.

Through the structural analysis, the scientists found new textures beneath the painting's veneer. They were able to uncover the thickness of paint strokes, the order of the layers of paint, and even wrinkles in the canvas from pigment deterioration, or mechanical tension on the canvas.

Then, at the very bottom of the painting, the team found a signature. Blind to the visible eye, to X-ray, and to infrared analysis, the researchers identified what they believed to be Goya's signature in the image created by the terahertz reflections. The researchers report that the signature was likely written with a pencil. They suggest that, over the years, the top coat of varnish darkened and obscured the carbon signature. They write that the signature might be missing in 2007 X-ray studies of the painting because carbon has a very similar atomic weight to that of the canvas and paint pigments, leaving the X-ray imaging unable to distinguish between the materials.

Identifying chemical compositions of paint pigments and other paint media remains an area the researchers hope to explore. In their study, the team found that some pigments reflected terahertz waves more than others. They speculated that these have a higher metallic content leading to higher reflectivity. In the future, the researchers imagine that the creation of a pigment spectroscopy database could enable artists, restorationists, and historians to study the detailed chemical compositions of paintings.